The present invention relates to cemented carbide buttons useful in tools for rock drilling, mineral cutting, oil drilling and in tools for concrete and asphalt milling.
In U.S. Pat. No. 4,743,515, cemented carbide buttons are disclosed with a core of finely and evenly distributed eta-phase embedded in the normal alpha+beta-phase structure, and a surrounding surface zone of only alpha+beta-phase. (alpha=tungsten carbide, beta=metal binder, e.g., Co, and eta=M.sub.6 C, M.sub.12 C and other carbides, e.g., Co.sub.3 W.sub.3 C). In the inner part of the surface zone situated close to the core of that body, the Co-content is higher than the nominal content of Co. The Co-content in the outermost part of the surface zone is lower than the nominal and increases in the direction towards the core up to a maximum usually at the eta-phase core.
Cemented carbide buttons according to the mentioned patent have given increased performance for all cemented carbide grades normally used in rock drilling.
When drilling with buttons according to the above-mentioned patent, the Co-poor surface layer is successively worn away. The Co-rich intermediate layer, when exposed, is worn more rapidly than the surrounding areas and a crater is formed (FIG. 1.3). As a result, the risk for spalling is increased and at the same time the drilling rate is decreased. At continued wear, the eta-phase core is exposed and the button then assumes a more rounded cap shape, FIG. 1.5. The wearing through of the Co-rich intermediate zone is particularly critical in rotary crushing drilling with chisel shaped or conical buttons which are not reground. In order to avoid too deep a crater in the button, the thickness of the eta-phase free surface zone is kept to a minimum. The risk is then that the Co-poor surface zone peels off and exposes the Co-rich part with a resulting rapid wear. The button thereby quickly loses several mm in protrusion height. The protrusion and shape of the button influence the drilling properties, in particular the penetration rate.